Summary of Work: Magnetic resonance imaging techniques provide the opportunity to visualize internal organs at microscopic resolution in live animals and to conduct specialized studies on fixed blocks of pathologic specimens. Techniques are being refined for use of hyperpolarized helium in imaging fine structures of the lung in live animals. Initial images show fine detail of branched airways down to the level of alveolar structures. Images of fixed liver specimens from rats treated with the hepatotoxicants bromobenzene allow definition of hepatic lobules and show tissue alterations at low doses where conventional histopathology does not detect lesions. Past work has demonstrated that MR imaging allows the visualization of liver tumor growth over time in rats treated with known hepatocarcinogens. These and similar studies show great promise as a means of monitoring dynamic tissue changes in live animals following exposure to potentially toxic environmental chemicals and allow yet another tool for the toxicologic pathologist to study tissue alterations.